API 520 D.2.2 (Sub-cooled liquid)

[chemengbr]

While creating an automated spreadsheet to perform sub-cooled PSV sizing,I noticed that per API 520, equation D.9 can be used for all cases highlighted in section D.2.2 (multi-component systems with nominal boiling range less than 150°F or single-component systems far from the critical point, and for multi-component systems with nominal boiling range greater than 150°F or single-component systems near the thermodynamic critical point). Therefore, I selected to use equation D.9 in my sheet due to its flexibility.

However, when I try to replicate the propane example from this section using equation D.9 I get a very different omega_s than the one API 520 obtained using equation D.8 (omega_s = 8.515). I am assuming I might be making a mistake while estimating the rho_9 (density evaluated at 90% of the saturation (vapor) pressure corresponding to the PSV inlet temperature (60F).

So, I am using chemcad to do an isentropic flash. My feed stream is at 60F and 300.7 psi (set pressure of 260psig + 10% overpressure +14.7 psi to get to psia). My flash in then isentropic at 96.8 psia, which is 90% of saturation pressure for propane at 60F. As a result of the flash I am getting a density of 31.38, which gives me an omega_s of ~0.15, so not even close to the 8.515 shown in API 520. Am I making a mistake with my flash?

 

[apetri]

the paper "A generalized correlation for one-component Homogeneus Equilibrium flashing choked flow" by Leung 1986, introduces the Omega method, the API version adopts a 2 points evaluation but still it remains a simplified model with the limits of HEM (vapor and liquid at equilibria, traveling at same speed etc.) plus a simplified correlation for properties, for a more flexible procedure API suggests the direct integration method, there are many threads in this forum discussing in details the method, see for example

https://www.eng-tips.com/viewthread.cfm?qid=329093

where a Excel page with VBA code is provided,
however take care that for flashing subcooled liquids HEM can give conservative results,
for these cases there are alternatives based on HNE, HNE-DS etc. methods,
if you prefer spreadsheets, there is an Excel page included in Prode Properties distribution to compare results of HEM vs HNE method

 

[chemengbr]

Thanks apetri, I'll look into some of the other methods.

As it turns out the source of difference in my omegas was due to user error (shame on me). In chemcad I was using a flash unit op with two outlet streams one vapor and one liquid. I was then combining the streams in excel to get my density, and as it turns out I was copying the wrong data over. I caught this mistake by only allowing one product stream out of the flash vessel, that way the density was automatically calculated. Once I fixed this my omegas were a lot closer.